The electrochemical N₂ reduction reaction (NRR) represents a green and sustainable route for NH₃ synthesis under ambient conditions. However, the mechanism of N₂ activation in the electrocatalytic NRR remains unclear. Herein, we found that the high spin state Mn³⁺-Mn³⁺ pairs induced by oxygen vacancy in MnO₂ nanosheets greatly enhance the catalytic activities. The strong electron transfer between d orbital of Mn and orbital of N₂ forces the N₂ to be of radical nature, which activates the hydrogenation process and weakens the N≡N bond. Based on the density functional theory (DFT) calculation results, we precisely designed mesoporous MnO₂ nanosheets with rich oxygen vacancies via using methyltriphenylphosphonium bromide (MPB) to induce more Mn³⁺-Mn³⁺ pairs (Mn^3-3-MnO₂), which can achieve a fairly high ammonia yield of up to 147.2 µg·h⁻¹·mg_cat⁻¹_. at −0.75 V vs. reversible hydrogen electrode (RHE) and a high Faradaic efficiency (FE) of 11%. Furthermore, these mesoporous MnO₂ nanosheets exhibit the superior durability with negligible changes in both NH₃ yield and FE after a consecutive 6-recycle test and the current density electrolyzed over a 24-hour period. Our findings offer an approach to designing highly active transition metal catalysts for electrocatalytic nitrogen reduction.